X-ray and optical observations of 1RXS J154814.5-452845: A new intermediate polar with soft X-ray emission

We report the identification of the ROSAT all-sky survey source 1RXSJ154814.5-452845 as new intermediate polar and present the results from follow-up optical and X-ray observations. The source shows pulsations with a period of 693 s both in the optical and X-ray light curves and the detection of a synodic frequency strongly suggests that this is the rotation period of the white dwarf. Although the one day aliasing and the sparse optical data coverage does not allow to unambiguously identify the orbital period, the most likely values of 9.37 h and 6.72 h add 1RXSJ154814.5-452845 to the intermediate polars with the longest orbital periods known. The optical spectrum displays features from the late type secondary and shows the presence of broad absorption lines at Hbeta and higher order Balmer lines which may be a signature of the white dwarf atmosphere, very similar to V709 Cas (RXJ0028.8+5917 Bonnet-Bidaud et al. 2001). The average X-ray spectra as obtained by the EPIC instruments on board XMM-Newton show hard emission typical for this class of objects but also the presence of soft blackbody-like emission similar to that seen from soft intermediate polars and thought to arise from the white dwarf surface heated by the hard X-rays. The best fit model comprises thermal emission from multi-temperature plasma in collisional ionization equilibrium with a continuous temperature distribution up to a maximum of similar to60 keV, an Fe fluorescence line at 6.4 keV and with equivalent width of 260 eV and a blackbody component with kT of 86 eV. The hard X-ray emission is absorbed by matter covering 47% of the X-ray source with an equivalent hydrogen density of similar to10(23) cm(2). The remaining hard emission is absorbed by a much reduced column density of 1.5 x 10(21) cm(2) as is the soft blackbody emission. Pulse-phase spectroscopy around spin maximum and minimum reveals that the flux variations are mainly caused by a change in the temperature distribution with higher intensity (a factor of similar to3 in the 1 keV emission) seen from the lower temperature plasma during spin maximum. The absorption in the high column density matter only decreases marginally during spin maximum. The emission characteristics are consistent with the accretion curtain scenario and features in the X-ray pulse profiles indicate that we observe one pole of the white dwarf and our line of sight is nearly parallel to the curtain at spin minimum while at maximum we have a more direct view to the cooling post shock accretion flow.